Generally, blades on a fan wheel have specially designed air flowing angles, so that air flows through upper and lower surfaces of the blades on a rotating fan wheel at different speeds due to different lengths of the upper and the lower blade surfaces. Air at the upper blade surfaces flows at a higher speed and accordingly has smaller pressure relative to the ambient air. On the other hand, air at the lower blade surfaces flows at a lower speed and accordingly has higher pressure relative to the ambient air. The pressure difference between the upper and the lower blade surfaces causes the air at the lower blade surfaces to push against the air at the upper blade surfaces to thereby produce an ascending force. A reaction force of the ascending force forms an airflow thrust. The airflow passes the blade surfaces and turns to produce the effect of doing work and accordingly, show the features of a fan.
However, conventional fans with rotor blade/stator structure or with single rotor blade and ribbed fan frame would usually produce relatively big wideband noise and narrow-band noise due to mutual influence between the wing-like shape of the fan blade structure and the fan frame. Further, conventional fans with wing-shaped blades usually produce forward airflow and therefore could not provide sufficient heat dissipation effect on heat-producing elements located behind the fans.
It is therefore tried by the inventor to develop an improved fan structure that is able to overcome the problems and disadvantages of the conventional fan structures.